(a) Field of the Invention
The present invention relates to a method for the continuous preparation of a polyethylene material having high strength and high modulus of elasticity. More specifically, the present invention relates to a method for effectively preparing a polyethylene material having high strength and high modulus of elasticity by mixing a specific ultra-high-molecular-weight polyethylene powder with a specific liquid organic compound, then continuously compression-molding the mixture at a temperature less than a melting point of the mixture, and rolling and drawing the same.
(b) Description of the Prior Art
The so-called ultra-high-molecular-weight polyolefins having noticeably high molecular weights are excellent in impact resistance and wear resistance and have self-lubricating properties, and therefore they are used as characteristic engineering plastics in many fields. This ultra-high-molecular-weight polyolefin has a much higher molecular weight than a usual polyolefin, and thus it is expected that if the ultra-high-molecular-weight polyolefin can be orientated at a high level, a molded article having high strength and high modulus of elasticity will be obtained. Accordingly, various researches have been conducted on techniques of the high orientation.
However, the ultra-high-molecular-weight polyolefin has a higher melt viscosity than the usual polyolefin, and so presently the ultra-high-molecular-weight polyolefin cannot be molded successfully in an ordinary manner, and it is also impossible to accomplish high orientation by drawing.
Pole Smith, Bieter Yarn Remstora et al. have suggested the method of preparing a fiber having high strength and high modulus of elasticity by drawing, at a high magnification, a gel obtained from a decal in solution (dope) of an ultra-high-molecular-weight polyolefin (G.B. Laid-open No. 2051667). However, in this dope, the concentration of the polymer having a weight average molecular weight of 1,500,000 is as low as 3% by weight, and that of the polymer having a weight average molecular weight of 4,000,000 was also extremely low, 1% by weight. Therefore, in the practice of the above-mentioned method, a large amount of a solvent is used, which means that the suggested method is very inconvenient from economical viewpoints such as a preparation way and handling of the high viscous solution.
Furthermore, various suggestions have been made with regard to methods for the high drawing and high orientation of single crystal mats of the ultra-high-molecular-weight polyolefins [EP Publication No. 115192, Japanese Patent Laid-open Publication Nos. 15120/1985 and 97836/-1985, Kobunshi Gakkai Yokoushuu, Vol. 34, No. 4, p 873 (1985) etc.].
In these methods, however, the ultra-high-molecular-weight polyolefin is first dissolved in a solvent such as xylene, decalin or kerosine to prepare a dilute solution, the latter is then subjected to cooling and isothermic crystallization so as to form a single crystal mat, and the latter is further subjected to solid phase extrusion and drawing. Therefore, also in this method, there is still the problem that a considerable amount of the solvent must be used at the time of the formation of the single crystal mat.
For the purpose of solving the above-mentioned problem, the present inventors have suggested a method in which an ultra-high-molecular-weight polyolefin powder is compression-molded at a temperature less than the melting point of the powder without dissolving and melting it, followed by rolling and drawing, in order to obtain a polyolefin material having high strength and high modulus of elasticity (Japanese Patent Laid-open Publication No. 41512/1988 and EP Publication No. 253513).
As a result of further investigations, the inventors have found that the polyolefin material having high strength and high modulus of elasticity can be manufactured in a high production efficiency by continuously compression-molding an ultra-high-molecular-weight polyolefin powder through a specific device (at a temperature less than the melting point of the polyolefin powder), and then rolling and drawing the molded material. As for this technique, a patent application has already been filed (Japanese Patent Application No. 320401/1988).
As discussed above, the present inventors have suggested the means for continuously compression-molding an ultra-high-molecular-weight polyolefin powder under a relatively low pressure so as to form a sheet, and according to this method, the rolling of a subsequent process can work effectively, so that in the rolled sheet, the total draw ratio inclusive of a roll ratio can be enhanced, which largely contributes to the improvement of quality and the increase of productivity. In this method, however, the fine uniformity of pressure takes place sometimes in the compression molding step and this portion appears as a defect, and the pressure is released through opposite end portions of the sheet. For these reasons, Yield is poor, and production efficiency is also low. In such a defect portion in the compression-molded sheet and such ununiformed end portions of the sheet, rolling cannot be effected uniformly. If there is the defect portion in the sheet, a bank is formed inconveniently at the time of rolling, which deteriorates productivity and yield noticeably. In addition, when the sheet including the defect portion is drawn in a subsequent step in the continuous process, the draw ratio cannot be increased sufficiently and the yield is low.